1. Field of the Invention
This invention relates to detection of trace chemical species in a gaseous sample and more particularly to detection of species using phase shift cavity ring down absorption spectroscopy.
2. Background Information
Nitrogen oxide species (NOx), in general, are important in determining the photochemistry of the earth's atmosphere, controlling the formation of tropospheric ozone, affecting the concentration of the hydroxyl (—OH) radical and contributing to acidic precipitation. Nitrogen dioxide (NO2), specifically, is formed by reaction of NO, a primary air pollutant produced during fossil fuel combustion from both stationary and mobile sources, with ozone and is converted back to NO by photolysis on a time scale of a few seconds to several minutes. In urban areas, high nitrogen dioxide concentrations can produce significant health effects on human populations, lead to photochemical “smog” formation, and decrease visibility due to secondary aerosol formation. In the U.S., nitrogen dioxide is regulated by a national ambient air quality standard (NAAQS) under the 1970 Clean Air Act. Detection and measurement of NO2 and other chemical species is quite important in monitoring and controlling such pollution. In addition, detection of nitrogen-based compounds and other species is becoming more desirable in the search for controlled substances, chemical poisons and explosive materials in transportation and public areas.
Currently, the most widely used technique for the measurement of nitrogen dioxide involves reducing it to NO using a heated molybdenum catalyst, followed by detecting the chemiluminescent reaction of NO with ozone. This technique is capable of achieving sensitivities in the sub-part-per-billion range and has historically remained the system-of-choice in air pollution and other atmospheric studies because of its reliability and comparatively low cost. However, this technique has been shown to be prone to interference from other nitrogen/oxygen-containing atmospheric trace species, such as peroxy nitrates and alkyl nitrates. As a result, the “measured” concentration of nitrogen dioxide can be considerably higher than the actual concentration.
It has been recognized that laser-based absorption and induced fluorescence techniques provide sufficient sensitivity and specificity to alleviate the problem of interfering species. Field-ready systems employing these techniques are, however, quite expensive and tend to require highly competent personnel to operate them. For example, two such techniques, cavity attenuated phase shift spectroscopy (CAPS) and cavity ringdown laser absorption spectroscopy (CRDS), employ a coherent light source, such as a laser to detect ambient gases within a test cavity defined by two or more mirrors that minimize optical loss. A detector is placed adjacent to one of the mirrors. Given the low loss, highly-reflective characteristic of the cavity, injected photons make many passes through the space of the cavity before slowly “leaking out” and traveling to the detector. For a given mirror reflectivity R the average number of round trips n made by a photon within the 2-mirror cavity is expressed as:N=R2/(1−R2).
Note that for R=0.9998 (readily obtainable now because of the development of the cavity ringdown technique), a cavity of 0.5 meter in length produces an effective path length of over 1 km. Referring to the graph 100 in FIG. 1, if a sine wave modulated continuous light source is coupled into the optical cavity, the resulting waveform 102 reaching the detector (shown as γ1 in FIG. 1) will be shifted in phase from the original waveform 104, a change which is readily measured with high accuracy using a lock-in amplifier. A square wave modulated light source may also be employed with similar results. Note that in this example, the modulation frequency is generally chosen so that γ1=45°. The presence of a gaseous species within the cavity adds another loss mechanism that competes with leakage of light through the opposing mirrors resulting in the curve 106. This additional loss term contributes to a change in the magnitude of the detected phase angle (γ2) of the modulated light, which allows one to detect the species of choice. In other words, the greater the absorbance of the gas in the sample cell, the faster the energy stored in the cavity decays, causing the measured phase shift, Δγ (defined as γ1-γ2) to increase. The nominal change in phase shift, ignoring the fact that the mirror reflectivity, R and gas absorption, A, vary as a function of wavelength, can be expressed as:Δγ=45°-arctan[((1−R)2(1−A)2)/(1−R2(1−A)2)]
For convenience, the phase shift has been defined as a positive for increased light absorption.
More particularly, systems based on either phase shift cavity ring down absorption spectroscopy and time decay cavity ringdown spectroscopy have typically coupled coherent light sources, i.e., lasers, with the resonant cavity so as to excite only a few modes of the cavity. To do so, the cavities have been designed to act similarly to laser cavities in which the radius of curvature of the cavity mirrors is long compared to the cavity length. This arrangement necessarily leads to low optical throughput when a spatially incoherent light source is employed. Furthermore, the use of coherent light sources necessarily entails complex apparatus in order to maintain the laser frequency at the resonant frequency of the cavity.
One particular problem with the use of a coherent light source is that all axial and transverse modes of the resonant optical cavity may not be equally excited. If the gas absorption shows any dependence on optical frequency at the scale of the free spectral range of the cavity, changes in cavity length on the order of a fraction of a wavelength of the light used cause measurable changes in the phase shift at constant absorber concentration. In general, it is difficult to maintain a laser frequency at the resonant frequency of the cavity, particularly where slight variations in cavity length may occur due to external forces and temperature variations. Overall, the deficiencies of a coherent light source-based cavity add significantly to the expense and complexity in the setup and maintenance of the apparatus
Resonant optical cavities designed to employ incoherent light sources have been designed. However, these designs are also limited in function and employ costly components. In one arrangement the output of a CW xenon arc lamp is modulated using an external photoelastic modulator—as this lamp cannot be effectively modulated by simply clocking the driving current input. The wavelength of light allowed to enter the resonant cavity is then selected with a Michelson interferometer posed between the lamp and cavity. The light leaking from the cavity is detected using a lock-in amplifier. A useable spectrum that is indicative of the species is obtained by incrementally step-scanning the interferometer's mirror. Clearly, this incoherent light technique involves sensitive and costly equipment that must be set up and maintained by a skilled technician.
It is desirable to provide an improved technique for detecting NO2 and other chemical species that maintains the advantages of quick detection time, species selectivity and concentration sensitivity of coherent-light-source CAPS without the limitations imposed by the use of a laser or other coherent light source or complex interferometer-based, incoherent-light driven cavity.